1. Technical Field
This disclosure relates in general to using plasma to reduce the viscous drag exerted by turbulent mixing vortices on an object moving through a fluid. This disclosure relates in particular to generating plasma between and above physical riblets and to generating plasma flow to create virtual riblets on the surface of an object moving through fluid to reduce the drag on the object.
2. Description of Related Art
A friction-reducing texture known as “riblets” may be used on the surface of an object passing through a fluid to reduce the friction drag on the object. Riblets are V-shaped grooves having a peak or ridge, and a valley. The sides of the valley are at an angle relative to the surface of the object. Riblets always extend along the aerodynamic surface in the direction of fluid flow. The V-shaped grooves are on the order of 10's to 100's of microns across and are used to modify the turbulent boundary layer structure. The riblet peaks are spaced closer than the dimensions of the turbulent mixing vortices and therefore force the vortices away from most of the exposed (or “wetted”) surface area. Thus the wetted area between the peaks is protected from the vortices and experience low local friction. The vortices do exist close to the riblet peaks and the viscous drag is high in the area near the peaks. The shape of the riblet may increase the surface area of the object by 40%. The combination of increased surface area and the remaining viscous drag at the peak of the riblet results in a reduction of viscous drag of just 6%.
The height of the riblets and the spacing between the riblets are usually uniform and on the order of 0.001 to 0.02 inches for most applications. Dimensionless units, sometimes referred to as wall units, are conventionally utilized in describing fluid flows of this type. The wall unit h+ is the non-dimensional distance away from the wetted surface or more precisely in the direction normal to the surface, extending into the fluid. Thus h+ is a non-dimensional measurement of the height of the riblets. The wall unit s+ is the non-dimensional distance tangent to the local surface and perpendicular to the flow direction, thus the non-dimensional distance between the riblets. In the prior art riblets, h+ and s+ are in the range between 10 and 20.
Under special circumstances, flow near the leading edge of a smooth surface moving through a fluid may be laminar. Laminar flow is fluid flow above the surface that is layered and steady. The boundary layer is a layer of fluid wherein the velocity of the fluid, relative to the surface, transitions from zero at the surface to the full velocity of the fluid in the free stream above the surface. As the fluid moves along the surface in a downstream direction, viscosity acts on the fluid causing the boundary layer to thicken as it is slowed by internal friction. When instabilities in the boundary layer create turbulent mixing vortices, the laminar boundary layer becomes a turbulent boundary layer. The turbulent flow in the boundary layer increases mixing and results in higher viscous forces between the fluid and the surface. Flow separation occurs at a point where the boundary layer separates from the surface. Turbulent boundary layers cause the vast majority of skin friction on vehicles in applications such as aircraft, trains, and automobiles, but successful means of reducing the skin friction associated with turbulent boundary layers has been historically elusive.